The marine natural product latonduine A ( 1 ) shows F508del-cystic fibrosis transmembrane regulator (CFTR) corrector activity in cell-based assays. Pull-down experiments, enzyme inhibition assays, and siRNA knockdown experiments suggest that the F508del-CFTR corrector activities of latonduine A and a synthetic analogue MCG315 ( 4 ) result from simultaneous inhibition of PARP3 and PARP16. A library of synthetic latonduine A analogs has been prepared in an attempt to separate the PARP3 and PARP16 inhibitory properties of latonduine A with the goal of discovering selective small-molecule PARP3 and PARP16 inhibitory cell biology tools that could confirm the proposed dual-target F508del-CFTR corrector mechanism of action. The structure activity relationship (SAR) study reported herein has resulted in the discovery of the modestly potent (IC 50 3.1 μM) PARP3 selective inhibitor (±)-5-hydroxy-4-phenyl-2,3,4,5-tetrahydro-1 H -benzo[ c ]azepin-1-one ( 5 ) that shows 96-fold greater potency for inhibition of PARP3 compared with its inhibition of PARP16 in vitro and the potent (IC 50 0.362 μM) PARP16 selective inhibitor (±)-7,8-dichloro-5-hydroxy-4-(pyridin-2-yl)-2,3,4,5-tetrahydro-1 H -benzo[ c ]azepin-1-one ( 6 ) that shows 205-fold selectivity for PARP16 compared with PARP3 in vitro . At 1 or 10 μM, neither 5 or 6 alone showed F508del-CFTR corrector activity, but when added together at 1 or 10 μM each, the combination exhibited F508del-CFTR corrector activity identical to 1 or 10 μM latonduine A ( 1 ), respectively, supporting its novel dual PARP target mechanism of action. Latonduine A ( 1 ) showed additive in vitro corrector activity in combination with the clinically approved corrector VX809, making it a potential new partner for cystic fibrosis combination drug therapies.